This invention relates to a process for crushing soft limerock and separating the crushed rock into different particle size ranges in order to produce screenings used in asphalt mix for paving roads.
Limerock is a variety of limestone which is mined in sections of the southeastern portions of the United States, principally in Florida. Limerock is crushed and sized for use in various construction operations e.g. aggregate for concrete structures, roadbeds, drainage fields, screenings for use in asphalt paving compositions, etc. In general, limerock has been classified as hard or soft based upon an abrasion test well known to those skilled in this art wherein particles are subjected to a specific process of abrasion and the amount rubbed away is weighed to provide a number representing the percentage of weight loss due to abrasion. If that percentage is 45 or less the particles are known as "hard limerock" and if the amount lost by abrasion is greater than 45% the particles are known as "soft limerock". Hard limerock has been the preferred material for use in concrete; asphalt aggregates, drainage field aggregates, and has heretofore been the only source for screenings to be used in asphalt mixes for paving purposes.
Soft limerock has heretofore not been acceptable for use as screenings in paving operations, because soft limerock had been considered to be too soft and could not be economically sized to meet the specifications required by the federal, state and local government for screenings used in roadbuilding and for other paving uses. The present standards of the State of Florida for roadbuilding purposes is that screenings from crushed limerock must have the following sized specifications:
100% passing 3/8 inch screen; PA0 85-100 percent passing 4 mesh screen (0.187 inch opening); and PA0 0-15% passing 200 mesh screen (0.0029 inch opening).
These specifications were not able to be met by prior art methods of sizing crushed soft limerock even though the crushed material would serve the purpose from the point of view of stability, wearability, etc. Furthermore the use of soft limerock would materially reduce the cost involved in roadbuilding since such material is derived from sources which are much less valuable than a similar size from hard limerock sources.
It is an object of this invention to provide a process for crushing and sizing soft limerock so as to produce screenings which will meet the requirements and specifications for use in asphalt paving operations. It is another object of this invention to provide a process for preparing several fractions of soft limerock having specific particle sizes. Still other objects will be apparent from the more detail description of this invention which follows: